The present invention relates to electronic assemblies, and more particularly, to connectors adapted for soldering to surface mount technology (SMT) circuit boards.
Connectors such as conductive pins and posts have long been used for electrically and/or mechanically connecting leads, terminals and electronic components to conductive traces or other devices mounted on printed circuit boards (PCBs) and in some cases, for connecting one PCB to an adjacent PCB. In recent years pins and posts have been developed for SMT applications. But problems arise in soldering one end of a pin to a PCB without a through-hole for holding the pin end. With SMT, pins are provided with a base or head that is placed on solder paste on a hole-less solder pad. However, during solder re-flow to form the solder bond, gasses may be generated. The gasses can produce voids in the solder bond which reduces strength of the bond. In addition, especially if excess solder paste has been screened onto the solder pad, which is not uncommon, the pin can undesirably move or xe2x80x9cskatexe2x80x9d laterally or tip-over before the solder hardens due in part to trapped gases generated by vaporized solder flux. Such skating results in misalignment problems between the pins and other connectors or circuit boards to which their upper ends are to be mated. On the other hand, if there is too little solder paste on the solder pad, the solder joint is often weak and must be re-enforced, particularly where parallel PCBs are to be later interconnected by a number of pins that extend perpendicularly therebetween.
The skating problem can be overcome by fixturing the pins in place during re-flow, but this adds cost and complexity to the assembly operation. However, this may not solve the problem of the gas-generated voids. One technique for strengthening the solder bond between a pin head and an underlying SMT circuit board involves a second soldering operation including the application of a relatively large volume of solder around and on top of the pin head, and re-flowing this large volume of solder to create a substantial fillet. This is time consuming and each additional step in the manufacturing process adds cost.
Therefore, it would be desirable to provide an improved pin geometry and SMT method that would be adapted for soldering to an SMT circuit board and would overcome the undesirable skating and void problems while at the same time enhancing the solder bond strength. However, any such improved pin geometry must allow reasonably inexpensive pin manufacture as well as permit the pin to be readily handled by automated pick and place machines that typically employ a pneumatic suction nozzle for lifting and depositing individual pins at precise locations on an SMT circuit board.
The present invention provides a surface mount connector for circuit board attachment in the form of an elongate pin with a head or base formed at the lower end of the pin. The base has a bottom surface that extends substantially normal to the axis of the pin and is dimensioned to be positioned on, and bonded to, a parallel-oriented pad on a primary circuit board, with solder or adhesive. The upper end of the pin is dimensioned and configured to be engageable with a mating connector or a secondary circuit board. The base is formed with at least one downwardly opening, laterally-extending channel that also opens through a peripheral wall of the base for permitting gases generated by vaporizing a solder flux during solder re-flow to escape from underneath the base. Exhaustion of the gases prevents undesirable skating of the connector and misalignment and eliminates voids in the solder bond that would otherwise weaken the solder bond. In addition, the channel increases the surface area contact between the solder and the base and provides a predictable degree of wicking, thereby further enhancing solder bond strength.
Preferably, the invention is applied to situations where individual pins are individually placed base down on the solder-paste-covered solid pad of the primary circuit board, because it is in that environment that the likelihood of undesirable bond voids and pin skating is increased.
Preferably, the bottom-side of the base comprises a plurality of intersecting channels that extend out to the periphery of the base, and preferably are symmetrically arranged on the bottom side as this configuration promotes outgassing that is also symmetrical which also tends to prevent pin fall-over during the soldering operation.
It is understood that SMT means that the board pad on which the SMT pin is mounted, but not adjacent parts of the board, is free of holes, and the pin does not have a section, usually called a Atail@, that enters a hole in the pad. A principal aim of SMT is to avoid the use of pins with a projecting section that is typically inside a hole in the board to maintain its position during the soldering process and to strengthen its mounting. Hence, a feature of the invention is the combination of a circuit board with at least one hole-free mounting pad on top of which is solder-bonded the base of an SMT component, the base being parallel to the mounting pad and comprising at least one channel extending out to the base periphery for the outgassing of gasses generated during the soldering process to reduce the tendency for the component to skate or undesirably move, rather than self-center, before the solder-bonding and to strengthen the bond.
The present invention also provides a circuit board interconnection method. The method first involves the step of forming a circuit board with at least one conductive pad. A layer of flux and solder paste is applied to the pad. The base of a connector is positioned on top of the solder paste. The bottom surface of the base is formed with a plurality of channels that are configured for capturing and venting vaporized flux gases. The solder paste is heated to re-flow the solder and cause the solder to wick into the channels. The solder is allowed to cool to form a solder bond between the base and the pad. Venting of the flux gases helps prevent undesirable skating or tip-over of the connector. It also increases the strength of the solder bond between the pad and the connector by eliminating voids and increasing the amount of surface of the base that is contacted by the solder than would otherwise be achieved with a flat base or a tubular base.